1. Technical Field
The present disclosure relates to Voltage-Controlled Oscillator (VCO) circuits and methods.
2. Background Information
Radio receivers and transmitters such as those in cellular telephone handsets typically employ local oscillators. Often times such a local oscillator involves a Phase-Locked Loop (PLL) that in turn employs a Voltage Controlled Oscillator (VCO). One way to realize a VCO involves an LC resonator whose natural oscillating frequency is adjusted to tune the frequency of oscillation of the VCO. One way to accomplish this tuning is to adjust the inductance value L of the resonator. Another way is to adjust the capacitance value C of the resonator. Switches such as field effect transistor switches can be employed to switch capacitors into and out of a capacitor network to adjust the capacitance value C of the resonator. Switches can also be employed to switch inductors into and out of an inductor network. Regardless of whether the switches switch in or out inductors or capacitors, the switches can be in the path of the oscillating current that passes back and forth between the inductance and the capacitance as the resonator resonates. If the switches are realized to be large transistor devices, then their series on-resistances are generally small. This is advantageous because power loss across the switches is advantageously small. Unfortunately, providing large switches generally introduces relatively large parasitic capacitances. Large parasitic capacitances are undesirable for several reasons. One reason is that the tunability of the resonator may be reduced. If, on the other hand, the transistor switches are made to be small devices, then the parasitic capacitances are advantageously smaller. Tunability of the resonator is improved but the series resistance of the switches is larger. As the oscillating current passes through the switches in their on states, there is power loss and noise is introduced into the oscillating signal. The quality factor Q of the resonator is reduced and phase noise in the output signal of the oscillator is increased. Making an LC resonator of a VCO tunable therefore generally has undesirable impact on VCO phase noise performance and power consumption.
In addition to the difficulties described above involving providing tunability, it is increasingly the case that a local oscillator should be tunable over a wide frequency range. It may, for example, be required that the receiver and the transmitter of a cellular telephone handset be usable to communicate using a selectable one of multiple different cellular telephone standards. The same handset may be required to communicate at a first time using the GMS standard, at another time using the WCDMA standard, and at a third time using the LTE standard. Due to having to operate using these various different standards, the local oscillators of the receiver and transmitter of the cellular telephone handset must generate signals that cover all of the frequency bands used by all of the standards supported. Such a local oscillator may, for example, be required to output a local oscillator signal in a wide tuning range where the lowest frequency is less than half of the highest frequency. This wide frequency range imposes further design difficulties on VCO design due in part to the need to provide large amounts of programmability in the capacitors and/or inductors of the VCO's resonator. Rather than providing this wide tunability range using one resonator, multiple resonators can be provided where the different resonators are fabricated to resonate at different center frequencies. VCO's involving multiple resonators are, however, generally inefficient and suffer from performance drawbacks. Such drawbacks include large area due to having to provide multiple inductors and include routing and layout difficulties.